The cloud, from which the star develops, is itself 500 times the mass of the Sun and many times more luminous and is the largest ever seen in the galaxy.

The researchers say their observations - to be published in the journal Astronomy and Astrophysics - reveal how matter is being dragged into the centre of the huge gaseous cloud by the gravitational pull of the forming star - or stars - along a number of dense threads or filaments.

Dr Nicolas Peretto, from Cardiff University, lead author of the report, said: "The remarkable observations from ALMA allowed us to get the first really in-depth look at what was going on within this cloud.

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"We wanted to see how monster stars form and grow, and we certainly achieved our aim.

"One of the sources we have found is an absolute giant - the largest protostellar core ever spotted in the Milky Way."

He added: "Even though we already believed that the region was a good candidate for being a massive star-forming cloud, we were not expecting to find such a massive embryonic star at its centre.

"This cloud is expected to form at least one star 100 times more massive than the Sun and up to a million times brighter. Only about one in 10,000 of all the stars in the Milky Way reach that kind of mass."

Different theories exist as to how these massive stars form but the team say their findings add weight to the idea that the entire cloud core begins to collapse inwards, with material raining in towards the centre to form one or more massive stars.

Co-author Professor Gary Fuller, from the University of Manchester, said: "Not only are these stars rare, but their births are extremely rapid and childhood short, so finding such a massive object so early in its evolution in our Galaxy is a spectacular result."

Dr Peretto said the team managed to get very detail observations usuing "only a fraction of ALMA's ultimate potential".

The observatory, which cost £850 million, is the most expensive and sophisticated in the world. It was funded by European, North American and east Asian astronomical agencies, of which Britain is a member.

It was built 5000m above sea level in the Atacama desert of northern Chile, one of the driest places on earth.